Such heat exchange devices are used on a large scale in many branches of industry, e.g. in the petroleum industry for cooling products obtained from hydrocrackers and reactors for partial oxidation of (hydro)carbon-containing fuels such as oil and coal and the like.
When for cooling purposes the hot gases are passed through tubes which are cooled with a cooling medium on the outside, the walls of the tubes acquire a high temperature owing to transfer of heat from the hot gases to the tube metal which heat is further transmitted to the cooling medium. Advantageously, for reasons of space saving helically coiled tubes are applied.
Dependent on the field of application, technical problems of different nature are met.
E.g. the cooling of hot gases obtainable from the gasification of (hydro)carbon-containing fuel, in which the presence of small solid particles is unavoidable, involves serious heat transfer problems and erosion/corrosion problems.
For example, hot synthesis gas produced by partial oxidation of (hydro)carbon-containing fuel is generally cooled in a heat exchanger located next to the gasifier thereby producing high pressure steam. A critical area is the gas inlet of the heat exchanger where the hot synthesis gas enters the heat exchange area. The wall thickness of the inlet area is to be minimised but should be thick enough to ensure mechanical integrity based on pressure and thermal loads. The gas velocity at the inlet area should be sufficiently high to prevent fouling but on the other hand low enough to ensure sufficiently low gas side heat transfer coefficients. In particular, obtaining an optimum between fouling and velocity is desirable.
EP-A-774103 describes an apparatus for cooling of hot gas wherein the inlet section is cooled by passing fresh cooling medium, i.e. water, along the exterior of the upstream end of the heat exchanger tubes. The flow of water is counter-current to the flow of hot gas within the tubes.
U.S. Pat. No. 5,671,807 discloses an apparatus for cooling of hot gas wherein the inlet section is cooled by passing fresh cooling medium, i.e. water, along the exterior of the upstream end of the heat exchanger tubes. The flow of water is co-current to the flow of hot gas within the tubes.
According to EP-A-774103 and U.S. Pat. No. 5,671,807 the inlet area is cooled by using fresh water also referred to as boiler feed water (BFW). By using fresh BFW a great temperature difference between the cooling medium and the hot gas and thus the desired low metal temperatures can be achieved. The quantity of the BFW as fed to the inlet section is however defined by the steam production of the unit. In order to obtain sufficient flow velocities at the heat transfer areas, small flow cross sections, the annular gaps around said upstream part of the heat exchanger tubes, are required. Such small annular gaps are a particular challenge in terms of design. In addition the equal distribution of the flow to the great number of tube inlets to be cooled is difficult to ensure.
A further disadvantage of these designs is when a sudden complete outage of the BFW flow occurs due to for example a failure. In such a situation the cooling of the inlet section will not be sufficient and damage may occur. In another situation the BFW flow may change as a result of the boiler level control modulating the BFW control valve. Especially in case of load increases of the hot gas passing the heat exchanger tubes the BFW control valve is initially shut off due to the increase of the steam bubble volume in the vessel before it is opened again for compensation of the increased steam production. In such a situation the inlet section is temporarily not sufficiently cooled.